update failure with BIG-IP 14.x and above

[ywei2017]

The virtual server update is broken with BIG-IP 14.x and above. See K27721265 and K13225405 for details.

Based on K13225405, PUT is a dangerous operation, as it leaves unspecified attributes to be reverted to the default -- instead of current values.

In my experiment:

• PUT is working for policy, monitor, pool, but broken for virtual server
• AT LEAST the virtual server, I get the HTTP 400 error.

I assume this can be fixed if the PUT is replaced with PATCH. Would you be open for a PR to fix it?